Electrode lens potential arrangement for a post-acceleration picture tube

ABSTRACT

An electron gun for a post-acceleration picture tube comprises a first electrode to which a low voltage is applied, a second electrode to which a high voltage is applied and a third electrode to which an intermediate voltage is applied. The main lens of the electron gun is composed of the first, second and third electrodes. The electron gun is particularly useful in a post-acceleration shadow-mask type color picture tube.

nite States Patent [191 Metro et a1.

[ 1 May8,1973

[54] ELECTRODE LENS POTENTIAL ARRANGEMENT FOR A POST- ACCELERATION PICTURE TUBE [75] Inventors: lchiro Ueno; Kazuo Toya, both of Tokyo,Japan [73] Assignee: Victor Company of Japan, Ltd., Yokohama, Japan [22] Filed: Jan. 27, 1971 [21] Appl. No.: 110,059

[30] Foreign Application Priority Data Jan. 30,1970 Japan ..45/8217 [52] US. Cl. ..315/l5, 315/31 TV, HOlj 29/46 [58] Field of Search ..3l5/14, 15, 16, 31 R,

[56] References Cited UNITED STATES PATENTS 2,227,033 12/1940 Schlesinger ..315/16 2,888,606 5/1959 Beam ..3l5/l5 X 3,082,342 3/1963 Pietri ....315/l5 X 3,185,879 5/1965 Evans, Jr. .3l5/14 X 3,437,868 4/1969 Ohkoshi et al. ..3l5/l5 3,586,900 6/1971 Seki et al .1 ..3l5/15 3,614,520 10/1971 Coleman ..315/15 Primary ExaminerCarl D. Quarforth Assistant ExaminerP. A. Nelson Att0rneyLouis Bernat [57] ABSTRACT 5 Claims, 9 Drawing Figures PATENTED W 3 sum 1 or Q FIG. 3 PRIOR ART INVENTORS [CH/R0 UEIvo BY fi T YH PATENIEDHAY 81973 3, 732,457

SHEET 2 UF 3 FIG. 4 PRIOR AT INVENTORS icmRo UENO BY KHZUO TOVH, 4 1

ELECTRODE LENS POTENTIAL ARRANGEMENT FOR A POST-ACCELERATION PICTURE TUBE This invention relates to an electron gun for a postacceleration picture tube and, more particularly, to an electron gun for a post-acceleration picture tube in which relationship between voltages applied to each grid electrode have been improved.

In a shadow-mask type color picture tube, generally, less than one third of the electron beams which are projected from electron guns can reach a surface of a phosphor screen. Therefore, the rate of utilization of the electron beams is very low. For this reason, it is difficult to obtain a color picture tube having a high luminance. It has then been proposed, and put to a practical use, to provide a post-acceleration picture tube in which a post-acceleration electric field is formed between the shadow-mask and the surface of the phosphor screen. In this post-acceleration picture tube, the electron beams pass through the shadow-mask at a sufficiently high rate to reproduce a relatively bright picture.

As electron guns used for this post-acceleration type of picture tubes, there have been a unipotential type electron gun and a bipotential type electron gun. Voltages applied to third and fourth grid electrodes in the unipotential type picture tube and voltage applied to a fourth grid electrode in the bipotential type picture tube are the same in magnitude as their shadow-mask voltages. In the post-acceleration picture tube, the shadow-mask voltage is much lower than the voltage over the surface of the phosphor screen for example, it maybe approximately half thereof. Accordingly, the aforementioned grid electrode voltages are very low.

In the conventional electron guns, the lower the electrode voltages, the less efficient are the electron guns. It is, therefore, difficult in the conventional electron guns to form sharp beam spots with little or no aberration on the phosphor screen, unless the quantity of electron beams from the electron guns is limited. If,

, however, the quantity of electron beams is limited to obtain a sharp reproduced picture it is then difficult to obtain a sufficiently bright reproduced picture.

It is, therefore, a general object of the invention to provide a novel and useful electron gun for a post-acceleration picture tube, eliminating the above described disadvantage of the conventional electron guns.

Another object of the invention is to provide an electron gun for a post-acceleration picture tube which is capable of reproducing a sharp and bright picture even at a relatively low electrode voltage.

A further object of the invention is to provide an electron gun, for a post-acceleration picture tube, which is capable of an excellent focusing action for a relatively large quantity of electron beams.

A still further object of the invention is to provide an electron gun for a post-acceleration picture tube in which consumption of deflection power can be minimized.

Other objects and features of the invention will become apparent from the description made hereinbelow with reference to the accompanying drawings, in which:

FIG. 1 is a vertically sectional side elevation of a post-acceleration shadow-mask type color picture tube, which is one example of a general post-acceleration picture tube, for which an electron gun according to this invention can be used;

FIG. 2 is a schematic diagram of an electron lens formed within the electron gun;

FIG. 3 is a schematic diagram showing a conventional unipotential electron gun;

FIG. 4 is a schematic diagram showing a conventional bipotential gun;

FIG. 5 is a schematic diagram showing one embodiment of an electron gun according to the invention;

FIG. 6 is a schematic diagram showing another embodiment of an electron gun according to the invention;

FIG. 7 is a diagram showing the loci of electron beams in the neighborhood of a main electron lens;

FIG. 8 is a diagram showing a relation between the diameter of a beam spot and a cathode current; and

FIG. 9 is a sectional perspective view showing one concrete example of the electron gun according to the invention.

First, as one example of a general post-acceleration picture tube for which the electron gun according to the invention can be used, a post-acceleration shadow mask type color picture tube will be explained with reference to FIG. 1. A color picture tube 10 comprises a glass bulb 11, provided with a deflection yoke 12, on the outside thereof. Within a neck portion 13 of the tube, there are enclosed three electron guns 15 mounted on a connecting base 14. Each of the three electron guns 15 is disposed at each vertex of a triangle formed around the axis of the tube. The bulb 11 comprises the neck portion 13, a funnel 16, and a face plate 17. On the inner surface of the face plate 17, there is provided a phosphor screen 18 which includes color dot phosphors of three colors, i.e., red, green and blue. A shadow-mask 19 is perforated with apertures corresponding to each dot trio and having a diameter larger than that of each phosphor dot on the phosphor screen 18. The shadow mask 19 is supported in parallel with and spaced from the phosphor screen 18. The funnel 16 is provided on its inner surface with a funnel anode 20.

A high voltage power source 21 applies voltages to the phosphor screen 18, shadow-mask 19 and anode 20, respectively, through wires 22, 23 and 24 which are connected to terminals 25, 26 and 27. The highest of these voltages is applied to the phosphor screen 18. A voltage applied to the shadow-mask 19 is about half of the voltage that is applied to the phosphor screen 18. Accordingly, a post-acceleration electric field is formed between the shadow-mask l9 and the phosphor screen 18. A voltage which is somewhat higher than the shadow-mask voltage is applied to the anode 20. Secondary electrons which are emitted by the beams from the electron guns striking the shadow-mask 19 are collected by the anode 20.

In the post-acceleration shadow-mask type color picture tube as described above, the necessary deflection power is reduced by setting the voltage on the shadowmask 19 lower than the voltage on the phosphor screen 18. Furthermore, the electrons pass through the shadow-mask 19 at rate which is increased by an electrostatic focusing by the shadow-mask l9 and the phosphor screen 18, whereby a bright reproduced picture is obtained.

The operation of the electron lenses of the electron gun used in the aforementioned picture tube will be explained with reference to FIG. 2. A cathode electron lens 30 is composed of an electrostatic field formed by a cathode 31, a first grid 32 and a second grid 33. A pre-focusing electron lens 34 is composed of an electrostatic field formed by the second grid 33 and a third grid 35. A main electron lens 36 is composed of an electrostatic field formed by an electrode system 37 including the third grid 35. An electron beam projected by the cathode 31 passes through the cathode lens 30, pre-focusing lens 34 and main lens 36 and forms a spot on the phosphor screen 18.

The conventional electron guns which have been used for the picture tube having the above described construction will be described.

FIG. 3 is a schematic diagram for illustrating the conventional unipotential type electron gun. This type of electron gun consists of the cathode 31, the first and second grid electrodes 32 and 33 and third, fourth and fifth grid electrodes 40, 41 and 42. To the third and fifth grid electrodes 40 and 42, a voltage is applied, via supply wire 45, a voltage which voltage is substantially of the same magnitude as the anode voltage or the shadow-mask voltage. To the fourth grid electrode 41, there is applied a voltage which is near a zero volt. The electrodes 40 and 42 are under the same voltage since they are connected to each other by a wire 43. To the electrode 41, there is connected a focusing voltage supply wire 44.

FIG. 4 shows the conventional bipotential type electron gun. The electron gun includes the cathode 31, the first and second grid electrodes 32 and 33, third and fourth grid electrodes 50 and 51. To the fourth grid electrode 51, a voltage supply wire 53 is used to apply a voltage which is substantially of the same magnitude as the anode voltage or the shadow-mask voltage. To the third grid electrode 50, there is applied a voltage which is about 16 percent to percent of the voltage applied to the electrode 51. To the electrode 50, there is connected a focusing voltage supply wire 52.

As described above, in the post-acceleration picture tube, the anode voltage or the shadow-mask voltage is maintained at about half the phosphor screen voltage, to form the post-acceleration electric field. Consequently, the voltages at the electrodes 40 and 42 in the unipotential type electron gun and the voltage at the electrode 51 in the bipotential type electron gun are maintained at a low voltage, which is about half the phosphor screen voltage. Since the voltages at the electrodes 40, 42 and 51 are relatively low, a sharp spot with little aberration cannot be obtained on the phosphor screen unless the quantity of the electron beam projected from the electron gun is limited. If the quantity of the electron beam projected from the electron gun is limited, a sufficiently bright picture cannot be obtained in the post-acceleration picture tube, although it has an advantage since the electron beam passes through the shadow-mask at a high rate.

In order to eliminate this disadvantage, it is conceivable to increase both the shadow-mask voltage and the phosphor screen voltage. According to this method, it is necessary to apply an increased shadow-mask voltage which is capable of effecting a sufficient focusing action for a greater quantity of electron beam. An even greater phosphor screen voltage is used to form the post-acceleration electric field with such increased shadow-mask voltage. If the electron beams strike the phosphor screen to which such a high voltage is applied, a large quantity of X-rays will be radiated from the phosphor screen. For this reason, this method cannot be put to a practical use.

This invention has eliminated the above described disadvantages of the conventional electron guns. Some preferable embodiments of the electron gun according to the invention will be described hereinbelow.

First, one embodiment of the electron gun according to the invention will now be described with reference to FIG. 5. In FIGS. 2 to 5, like reference numerals designate like component parts. The electron gun con sists of the cathode 31, the first and second grid electrodes 32 and 33 and third, fourth and fifth grid electrodes 60, 61 and 62. To the electrode 60, there is connected a focusing voltage supply wire 63. To the electrode 61, there is connected a high voltage supply wire 64. To the electrode 62, there is connected an inter mediate voltage supply wire 65. A low voltage is applied to the electrode 60, a high voltage to the electrode 61, and an intermediate voltage to the electrode 62, respectively. A compound convexo-concave electron lens is composed of an electric field formed by the electrodes 60 and 61, and a compound concavo-convex electron lens is composed of an electric field formed by the electrodes 61 and 62. The main electron lens 36 shown in FIG. 2 is composed of these compound electron lenses. The main electron lens thus composed is capable of effecting an excellent focusing action even for a large quantity of electron beam.

Performance of the electron gun according to the invention and that of the conventional electron gun will be compared with reference to FIGS. 7 and 8. In the description made hereinbelow, the electron gun according to the invention used for comparison is one shown in FIG. 5. The electrodes 60, 61 and 62 have the same diameter of 6.6 mm. The length of the electrode 61 is 1.6 times of its internal diameter. The length of the large diameter portion of the electrode 60 is 1.28

times its internal diameter, and the length of its small diameter portion is 3 mm. The interval between the opposing ends of the electrodes 60 and 61 and the interval between the opposing ends of the electrodes 61 and 62 are respectively 1 mm. The interval between the end of the large diameter portion of the electrode 60 and the opposing end of the electrode 33 is 2.1 mm. The conventional bipotential type electron gun which is used for comparison has the construction shown in FIG. 2 which lacks the electrode 62 of the electron gun shown in FIG. 5.

FIG. 7 is a diagram which shows the loci of electron beams in the neighborhood of the main electron lens, in the case where a main electron lens of approximately nine magnifications is used. Each curve in the diagram is drawn as a result of calculation made by an electronic computer using such values as the dimensions of each electron gun, distribution of the electrode voltages, and the measured value of incidence angle of the electron beam entering the main electron lens (diffusion angle of the electron beam emerged from the prefocusing lens). In this diagram, a curve Ia shows a locus of the electron beam, where a voltage of 1.625 KV is applied to the electrode 50 and 10 KV to the electrode 51 of the conventional bipotential type electron gun shown in FIG. 2 (hereinafter referred to as a KV bipotential type electron gun). Similarly, a curve Ila shows a locus of the electron beam, where 3.25 KV is applied to the electrode 50, and 20 KV to the electrode 51 of the conventional bipotential type electron gun (hereinafter referred to as a 20 KV bipotential type electron gun). A curve llIa shows a locus of the electron beam where a voltage of 4.57 KV is applied to the electrode 60 (the electrode 60 is made as a focusing electrode), 20 KV to the electrode 61, and 10 KV to the electrode 62, respectively, in the electron gun according to the invention shown in FIG. 5. A curve IV shows a locus of the electron beam where 4.57 KV is applied to the electrode 60, and 20 KV to the electrodes 61 and 62, respectively, in the electron gun according to the invention.

FIG. 8 shows a variation in a diameter of the electron beam spotrelative to a variation in a cathode current. The curves in FIG. 8 show results of measurement, by a microscope, of the diameters of the beam spots which occur when the phosphor screen is struck by the electron beams projected from electron guns, having a main electron lens of 11 magnifications. In the diagram, the variation in the diameter of the electron beam spot relative to the variation in the cathode current is shown by the curve lb with regard to the conventional 10 KV bipotential type electron gun. The curve IIb is drawn with regard to the conventional 20 KV bipotential type electron gun. The curve IIIb is drawn with regard to the electron gun according to the invention.

FIG. 8 shows that there is a difference in the diameter of the electron beam spot produced by each electron gun, even in a part where no aberration is supposed to exist within a range in which the cathode current is very small. This phenomenon is thought to have occurred due to the difference in virtual image magnification of the pre-focusing lens of each electron gun caused by the difference in the voltages applied to the electrodes 50 and 60 of each electron gun.

The principal concern of this invention is the main electron lens of the electron gun. There are generally various factors upon which performance of the main electron lens depends. In the unipotential type electron gun, the higher the voltage applied to the electrodes 40 and 42, the sharper will be the beam spot, with less aberration. Similarly in the bipotential type electron gun, the higher the voltage applied to the electrode 51, the sharper will be the beam spot. In the post-acceleration picture tube, however, the voltages applied to the electrodes 40, 42 and 51 had to be limited to a low level, as described above, so that it was impossible to use a large quantity of the electron beam.

In the electron gun according to the invention which has been illustrated with reference to FIG. 5, a high voltage (e.g. phosphor screen voltage of 20 KV) is applied to the electrode 61, an intermediate voltage (e.g. funnel voltage of IO KV) to the electrode 62 and a low voltage (e.g. a voltage corresponding to about 15 percent to 25 percent of the voltage applied to the electrode 61) to the electrode 60, respectively. By this voltage distribution, an electron gun can be obtained which has performance very similar to the bipotential or unipotential type electron gun with a high applied voltage.

The magnification M of the main electron lens of the electron gun is given by the following equation:

Where ()0 is the incidence angle of the electron beam entering the main electron lens, (is is the exit angle of the electron beam emerging from the main electron lens, V0 is the voltage at the incidence point where the electron beam enters the main electron lens (the voltage at the electrode in the present embodiment), and Vs is the voltage at the exit point where the electron beam emerges from the main electron lens (the voltage at the electrode 62 in the present embodiment).

As will be seen from the above equation, the lower the voltage Vs, the larger will be the magnification M and the diameter of the electron beam spot on the phosphor screen. It will be noted, however, that even if the voltage Vs becomes low, the magnification M can be maintained constant by increasing the angle 0s. The electron gun constructed according to the invention, makes use of this principle. In the electron gun according to the invention, the voltage Vc and the angle 00 are about the same as in the bipotential type electron gun, the voltage Vs is about half that in the bipotential type electron gun.

As will be seen from FIG. 7, the angle 0s is increased in this invention. The angle 0s can be increased without increasing the distance of the electron beam from the axis of the main electron lens because the electron beam is first diverged by a concave lens and then eonverged by a convex lens.

This will be further illustrated with reference to FIGS. 7 and 8. The incidence angle of the electron beam entering the main electron lens is determined by measuring the diffusion angle of the electron beam emerging from the pre-focusing lens. This diffusion angle becomes small as the voltage applied to the electrode 60 increases relative to the voltage applied to the electrode 33. The electron beam which has entered the main electron lens at a certain incidence angle is subjected to converging and diverging actions of the compound electron lens formed by the electrodes 60 and 61. When the voltages applied to the electrodes 61 and 62 are equal each other (in which case the electron gun becomes the bipotential type electron gun), the electron beam follows the locus shown by the curve IV. In this case, the electron beam does not converge on the phosphor screen, but it forms a virtual image.

If, however, the voltage applied to the electrode 62 is made lower than the voltage applied to the electrode 61, the electron beam is subjected to diverging and converging actions of the electron lens formed by the electrodes 61 and 62, and the beam follows the locus shown by the curve Illa. In this case, the electron beam converges on the phosphor screen and'forms a real image.

Aberration of the electron lens, particularly spherical aberration, increases in proportion to the cube of the distance of the electron beam, measured from the lens axis. Accordingly, the aberration is a great factor upon which the performance of the lens depends. The maximum distance from the lens axis of the curve Illa (the value at the peak P,,,) is almost equal to the maximum distance from the lens axis of the curve Ila (the valve at the peak P and the distance of curve Illa is much smaller than the maximum distance from the lens axis of the curve la (the value at the peak P,). It will be understood from this that, in the electron gun according to the invention, spherical aberration is produced at substantially the same level as in the electron beam of the bipotential type electron gun in which a voltage of 20 KV is applied to the electrode 51. This is shown by FIG. 8 in which the curve IIIb is very similar to the curve lIb. Therefore, it will be easily understood that the electron gun according to the invention is also excellent also in the spherical aberration.

In the electron gun according to the invention in which the phosphor screen voltage of 20 KV is applied to the electrode 61, the funnel anode voltage of 10 KV to the electrode 62, and the voltage of 3 KV to KV to the electrode 60, respectively, there is a performance equivalent to the conventional bipotential type electron gun, in which the funnel anode voltage of 20 KV is applied to the electrode 51 and the phosphor screen voltage is increased to 40 KV. If the electron gun, according to the invention, having the aforementioned voltage distribution, is compared with the conventional bipotential type electron gun in which the funnel anode voltage of KV is applied to the electrode 51 and the phosphor screen voltage is made KV, there is a difference in performance between the two electron guns. This difference is shown by the difference between the curve IIIa and the curve la in FIG. 7, and the difference between the curve IIIb and the curve lb in FIG. 8. In the curve lb, the diameter of the beam spot increases as the cathode current increases, more than the diameters shown in the curves Ilb and IIIb. This is because the aberration of the electron lens in the electron gun shown in the curve 1b is much larger than those in the electron guns shown in the curves Ilb and IIIb. The diameter of the electron beam spot in the curve IIIb is substantially the same as that in the curve lIb. This indicates that the electron gun, according to the invention, has a performance which is substantially equal to the performance of a 20 KVbipotential type electron gun, in respect of the diameter of the electron beam spot.

With reference to FIG. 7, a comparison will be made between the rising portions of the curves Ila and lIIa. It will be noted that the incidence angle of the electron beam in the curve Ila is different from the incidence angle in the curve Illa notwithstanding that the same voltages are applied to the electrodes 51 and 61. This is because the electron beam which follows the locus shown by the curve "la is focused by two electron lenses. In this case, the lens action of the electron lens formed by the electrodes 60 and 61 may be smaller than the lens action in case of the electron beam which follows the locus shown by the curve Ila. As a result, the voltage at the electrode 60 in the curve 111a is higher than the voltage in the curve Ila whereby the lens action of the pre-focusing lens in the curve Illa is larger than that in the curve Ila.

In the electron gun according to the invention, on both sides of the electrode 61 to which a high voltage is applied, there are disposed opposite to each other the electrode 62 to which an intermediate voltage is applied and the electrode 60 to which a low voltage is applied. When the electrodes are equal in diameter and different voltages are applied thereto, and when these electrodes are disposed opposite to each other, in-

fluence which one electrode has on other electrode extends to a point on its axis which is at a distance nearly equal to the diameter of the electrode. As a result, in the electron gun according to the invention, in order that the high voltage may be applied to the electrode 61, its length must be more than double its diameter, if it is to act sufficiently on the electron beam passing therethrough. This length assumes that each electrode has an equal diameter. On the other hand, if the electrode 61 becomes too long, it tends to cause a coma aberration. It is, therefore, desirable to select the length of the electrode 61 at such dimension that it has an excellent high voltage action on the electron beam and yet no coma aberration will occur. With due regard to this point, the length of the electrode 61 in the above described embodiment is preferably 1.6 times of its diameter. In this case, the maximum voltage on the axis of the electrode 61 is about 98 percent of the electrode voltage at the electrode 61, and little or no coma aberration is found as a result of the experiment.

Examples of the values of the voltages applied to each part are given hereinbelow.

Example I Example ll Phosphor screen voltage 20.0 KV 20.0 I(V Funnel anode voltage 10.0 KV 14.0 KV Shadow-mask voltage 9.9 KV 13.9 KV Voltage of electrode 62 I00 KV 14.0 KV Voltage of electrode 61 20.0 KV 20.0 KV Voltage of electrode 60 3.8 KV 3.2 KV Voltage of electrode 33 0.5 KV 0.5 KV Voltage of electrode 32 0.07 KV 0.07 KV In the above examples, the voltage of the electrode 32 is a cut-off voltage when the cathode voltage is O V.

FIG. 6 illustrates another embodiment of the electron gun according to the invention. In FIGS. 5 and 6, the same reference numerals are used to designate like component parts, and the description thereof will be omitted. A fourth grid electrode has a portion opposite to the electrode 60 which is of a reduced diameter. The reduced diameter portion 71 is inserted in the electrode 60. By this construction, a withstanding voltage between the electrode 60 and the electrode 70 becomes greater. Since the operation principle, performance etc. of the second embodiment are the same as the first embodiment illustrated in conjunction with FIG. 5, the description thereabout will be omitted.

FIG. 9 is a partly sectional perspective view of a concrete example of the electron gun schematically shown in FIG. 5. The first to fifth grid electrodes to 84 shown in FIG. 9 correspond to the electrodes 32, 33, 60, 61 and 62 shown in FIG. 5. A high voltage supply wire 85 corresponds to the wire 64. Three electrode assemblies, each of which consists of the electrodes 80 to 84, are combined together by a binding glass 86 and held in position. The three electrode assemblies and a radial convergence 87 provided in front thereof are received in a neckglass 88.

In the foregoing embodiments, the electron gun according to the invention has been described as adapted for a color picture tube. The electron gun, however, is applicable to a black-and-white picture tube. If it is used in a black-and-white picture tube, it has an advantage that a bright and sharp picture can be reproduced with a small deflection power even in case the voltage of the high voltage power source is low.

The invention is not limited to these embodiments but various modifications and variations may be made without departing from the spirit and scope of the invention.

What we claim is:

1. An electron gun for a post-acceleration picture tube which comprises a phosphor screen, means for applying a voltage to said phosphor screen, funnel anode electrode means, means for applying to said anode electrode means a voltage which is lower than the voltage applied to said phosphor screen, a shadow-mask, means for applying to said shadow-mask a voltage similar to the voltage applied to said anode electrode means; said electron gun comprising a cathode for projecting an electron beam, first, second and third electrodes arranged in series along the direction of travel of the electron beam projecting from said cathode, means for applying to said first electrode a voltage which is lower than the voltage applied to said anode electrode means and for actuating said first electrode as a focusing electrode, said first electrode being disposed nearest to said cathode of said three electrodes, means for applying to said second electrode a voltage which is substantially equivalent in its magnitude to the voltage applied to the phosphor screen, said second electrode being disposed in the middle of said three electrodes, and means for applying to said third electrode a voltage which is substantially equivalent in its magnitude to the voltage applied to the anode electrode means, said third electrode being disposed as the remotest electrode from said cathode of said three electrodes, said first, second, and third electrodes to which said voltages are applied by the voltage applying means forming a main electron lens responsive to an electrostatic field produced therebetween. I

2. An electron gun for a post-acceleration picture tube which comprises a phosphor screen, means for applying a voltage to said phosphor screen, funnel anode electrode means, means for applying to said anode electrode means a voltage which is lower than the voltage applied to said phosphor screen, a shadow-mask, means for applying to said shadow-mask a voltage which approximates in its magnitude said voltage applied to said anode electrode means; said electron gun comprising a cathode for projecting an electron beam, first, second, and third electrodes arranged in series along the direction of travel of the electron beam projecting from said cathode, means for applying to said first electrode a voltage which is lower than the voltage applied to said anode electrode means and for actuating said first electrode as a focusing electrode, said first electrode being disposed nearest to said cathode of said three electrodes, means for applying to said second electrode a voltage which is substantially equivalent in its magnitude to the voltage applied to said phosphor screen, said second electrode being disposed in the middle of said three electrodes, and means for applying to said third electrode a voltage which is substantially equivalent in its magnitude to the voltage applied to said shadow-mask, said third electrode being the electrode which is disposed remotest from said cathode of said three electrodes, said first, second, and third electrodes to which said voltages are applied by the voltage applying means forming a main electron lens responsive to an electrostatic field roduced therebetween.

3. The electron gun as efined lll claim 1 wherein said voltage applied to said third electrode is substantially half the voltage applied to said second electrode.

4. The electron gun as defined in claim 1 wherein said first, second, and third electrodes have the same diameter.

5. The electron gun as defined in claim 4 wherein the end portion of said second electrode opposite to the end portion of said first electrode has a diameter which is smaller than said same diameter, and a part of the end portion of said second electrode is inserted into the 

1. An electron gun for a post-acceleration picture tube which comprises a phosphor screen, means for applying a voltage to said phosphor screen, funnel anode electrode means, means for applying to said anode electrode means a voltage which is lower than the voltage applied to said phosphor screen, a shadow-mask, means for applying to said shadow-mask a voltage similar to the voltage applied to said anode electrode means; said electron gun comprising a cathode for projecting an electron beam, first, second and third electrodes arranged in series along the direction of travel of the electron beam projecting from said cathode, means for applying to said first electrode a voltage which is lower than the voltage applied to said anode electrode means and for actuating said first electrode as a focusing electrode, said first electrode being disposed nearest to said cathode of said three electrodes, means for applying to said second electrode a voltage which is substantially equivalent in its magnitude to the voltage applied to the phosphor screen, said second electrode being disposed in the middle of said three electrodes, and means for applying to said third electrode a voltage which is substantially equivalent in its magnitude to the voltage applied to the anode electrode means, said third electrode being disposed as the remotest electrode from said cathode of said three electrodes, said first, second, and third electrodes to which said voltages are applied by the voltage applying means forming a main electron lens responsive to an electrostatic field produced therebetween.
 2. An electron gun for a post-acceleration picture tube which comprises a phosphor screen, means for applying a voltage to said phosphor screen, funnel anode electrode means, means for applying to said anode electrode means a voltage which is lower than the voltage applied to said phosphor screen, a shadow-mask, means for applying to said shadow-mask a voltage which approximates in its magnitude said voltage applied to said anode electrode means; said electron gun comprising a cathode for projecting an electron beam, first, second, and third electrodes arranged in series along the direction of travel of the electron beam projecting from said cathode, means for applying to said first electrode a voltage which is lower than the voltage applied to said anode electrode means and for actuating said first electrode as a focusing electrode, said first electrode being disposed nearest to said cathode of said three electrodes, means for applying to said second electrode a voltage which is substantially equivalent in its magnitude to the voltage applied to said phosphor screen, said second electrode being disposed in the middle of said three electrodes, and means for applying to said third electrode a voltage which is substantially equivalent in its maGnitude to the voltage applied to said shadow-mask, said third electrode being the electrode which is disposed remotest from said cathode of said three electrodes, said first, second, and third electrodes to which said voltages are applied by the voltage applying means forming a main electron lens responsive to an electrostatic field produced therebetween.
 3. The electron gun as defined in claim 1 wherein said voltage applied to said third electrode is substantially half the voltage applied to said second electrode.
 4. The electron gun as defined in claim 1 wherein said first, second, and third electrodes have the same diameter.
 5. The electron gun as defined in claim 4 wherein the end portion of said second electrode opposite to the end portion of said first electrode has a diameter which is smaller than said same diameter, and a part of the end portion of said second electrode is inserted into the end portion of said first electrode. 